Present laser range finders use either the time of flight method for finding the distance to an object or a Radio Frequency (RF) modulated light signal wherein the phase of the transmitted and reflected light signals are compared to extract distance using the speed of light. The later method is more widespread since it is easier to implement. A factor that determines the distance measurement accuracy is the calibration means. There are many random variables that degrade accuracy such as temperature, component drift, and internal gain settings. To achieve 1 mm or better accuracy, frequent calibration is required. This requires that the laser range finder be self-calibrating. A requirement for inclusion in a mobile device (e.g., cell phones) is to have the range finder be highly integrated and use very little power.
An optoelectronic distance measurement system for use in a mobile device (e.g., mobile phone) includes a lens system, a laser diode emitting a high frequency intensity modulated measurement optical light signal, and at least one receiver photodetector for receiving the measurement light signal reflected from a measurement object surface. A small portion of the optical signal from the laser is coupled to a reference photodetector and mixed with a reference local oscillator frequency which has a small offset frequency from that of the laser modulation frequency to generate an intermediate frequency reference signal for phase measurement. The receiver photodetector converts the high frequency optical signal reflected from the measurement object surface to a high frequency electrical signal. The receiver high frequency photodetector signal is mixed in a high frequency demodulator with the reference local oscillator to generate an intermediate frequency signal while preserving the phase delay information. A microcontroller combined with a Analog-to-Digital Converter (ADC) measures the phase difference of the intermediate frequency between the laser transmitting reference or calibration signal and the received reflected signal. The phase difference between these signals is directly proportional to the distance to be measured factoring in the speed of light.
For a laser range finder in mobile phone applications, it is desirable that the laser emitting power is limited to the Food and Drug Administration Class 1. It is generally required to have laser optical power below 1 milliWatt in the visible spectrum range for continuous wave operation. In order to be able to fit into the form factor of a mobile phone, there should not be any mechanical moving part or parts in the laser range finder. The lens assembly is also limited by the dimensional limitations of a typical mobile phone. It is preferred that the diameter of the lens not exceed 4 millimeter, while the height of entire range finder assembly not exceed 3 millimeter.
Since the optical signal strength at the photodetector drops off at one over the square of the range distance (1/R2), the photodetector and subsequent amplifier should have significant dynamic range to ensure that the range finder has 1 millimeter resolution for a range from 10 millimeter to 10 meters. In addition, the surface optical reflectance of a measurement object is an unknown. A dedicated automatic gain control mechanism is needed to adjust the gain of the photodetector to ensure that there is a sufficient signal to noise ratio to recover the phase information.
In addition to recovering the optical signal, it is desirable for the range finder to have the phase information be consistent at different gain settings. It is also desirable to calibrate the photodetector and subsequent intermediate frequency filter's phase to ensure that the laser range finder operation be substantially independent of measurement object surface reflectance, optical component variations, and electronic component variation and aging effects.